If they _________ that the weather was going to be so bad, they wouldn't have gone to the beach.

The correct choice, "had known," is appropriate in this context because it utilizes the past perfect tense, which is necessary for expressing a hypothetical situation in the past. The phrase "If they had known that the weather was going to be so bad" indicates that the knowledge (or lack thereof) about the weather occurred before their decision to go to the beach. In this conditional structure, it implies that their actions would have been different had they possessed that knowledge.

This structure is typical of third conditional sentences, which describe unreal situations in the past, often indicating regret or reflection on what could have been different. It helps to establish a clear temporal relationship between the awareness of the weather and their subsequent decision.

In contrast, using "were knowing," which is not a grammatically correct form in this context, doesn’t convey a completed action and would disrupt the conditional statement's meaning. The choice "knew" reflects a simple past tense but does not fully capture the hypothetical aspect needed here. Lastly, "known" lacks a proper subject-verb structure and fails to function correctly on its own. Thus, "had known" is the only option that accurately conveys the intended meaning in a grammatically correct manner.